He was running a website with the explicit purpose of selling illegal goods and services
Did he explicitly require that activity for participation? Just because it was thought by some to be an optimal medium for that activity, doesn't mean it was there for that explicit purpose. It was a website using TOR. TOR was created by the military. Neither the inventor of websites, nor the inventor of TOR created them for the purpose of illegal activity. If he was a fan of that activity, that also doesn't make it any more illegal than the publications available under the protection of the First Ammendment. If he participated in the activity of selling illicit goods and/or services, that would be another matter. Besides, these goods and services are not illicit everywhere his website served. His biggest mistake was being a US citizen.
He doesn't have to require it.
It was clear however that he specifically encouraged it - hence why his operation was a criminal operation.
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And his crime wasn't being a US citizen, his crime was breaking US law.
Being outside this country does not mean you can run a narcotics website that accepts business inside the United States.
Why do you think so many gambling web sites blacklist U.S. IP addresses?
Because if they didn't, they would be violating U.S. law and could be arrested if they ever came here - whether or not they are US citizens. Hell they don't even have to come here to be arrested if the country they are in has an extradition treaty with the U.S.
Know the laws of the country you are doing business in. Don't do business in countries where your business is illegal.